The present invention relates to digital communications, and more particularly to a carrier recovery scheme for quadrature amplitude modulated (QAM) systems.
In order to communicate digital data via an analog channel, the data is modulated using, for example, a form of pulse amplitude modulation (PAM). Typically, quadrature amplitude modulation is used to increase the amount of data that can be transmitted within an available channel bandwidth. QAM is a form of PAM in which a plurality of bits of information are transmitted together in a pattern referred to as a "constellation," which can contain, for example, 16, 32, or 64 points, etc.
In pulse amplitude modulation, each signal is a pulse whose amplitude level is determined by a transmitted symbol. In 16-QAM, symbol amplitudes of -3, -1, 1 and 3 in each quadrature channel are typically used. In 64-QAM, symbol amplitudes of -7, -5, -3, -1, 1, 3, 5 and 7 are typically used in each quadrature channel.
A phase lock loop (PLL) is a common and well known method used to recover the carrier in signal demodulators. When used in such applications, the PLL is sometimes referred to as a carrier recovery loop (CRL). A free running oscillator is used to translate the input signal frequency to baseband, and a phase rotator may be used to recover the carrier phase. Examples of known carrier recovery loops can be found in A. Leclert and P. VanDamme, "Universal Carrier Recovery Loop for QASK and PSK Signal Sets," IEEE Transactions on Communications, Vol. Com-31 No.1, January 1983, pp. 130-136; H. Sari and S. Moridi, "New Phase and Frequency Detectors for Carrier Recovery in PSK and QAM systems," IEEE Transactions on Communications, Vol. 36, No. 9, September 1988, pp. 1035-1043; and U.S. Pat. No. 4,958,360 to H. Sari, entitled "Circuit for Recovering the Carrier in Digital Transmission Systems."
In the '360 patent, carrier recovery is accomplished using apparatus that selects received signal points with the aid of selection zones formed by ring segments situated around certain states of the signal constellation. The signals of the in-phase (I) and quadrature (Q) channels are used to address a memory that contains the information belonging to or not belonging to the signal point received in one of the selected zones as well as the amplitude and the error sign that relate thereto. An important feature of the system disclosed in the '360 patent is that the received signal points are selected with the aid of zones situated around only certain states of the signal constellation. The intent is to cancel all of the received signal points liable to cause a faulty acquisition and to use only those points that are liable to cause a correct acquisition. More specifically, in the 64-QAM embodiment illustrated in the '360 patent, the selection zones for the (+,+) quadrant are limited to areas within rings around diagonal states (1,1), (3,3), and (7,7). Equivalent selection zones are used in the remaining quadrants of the constellation.
A problem with the system provided in the '360 patent is that the gain of the system must be extremely accurate in order to effect carrier recovery. If the gain is not as expected, the received points may be outside of the defined selection zones and it will be impossible to acquire the signal. Thus, any kind of interference that can affect gain, such as echoes that can occur in terrestrial and cable television transmission, can render the system unusable.
It would be advantageous to provide a method and apparatus for tracking a digitally modulated signal during the carrier recovery process that is not affected by changes in the amplitude of the received signal, or the gain within the communication channel or the receiver itself. Such a system should be able to lock onto the carrier even under heavy interference and other detrimental effects, such as noise, echoes, irregular system gain and frequency drift.
The present invention provides a method and apparatus having the aforementioned advantages.